Embedded nonvolatile memory and forming method thereof

ABSTRACT

A nonvolatile memory embedded in an advanced logic circuit and a method forming the same are provided. In the nonvolatile memory, the word lines and erase gates have top surfaces lower than the top surfaces of the control gate. In addition, the word lines and the erase gates are surrounded by dielectric material before a self-aligned silicidation process is performed. Therefore, no metal silicide can be formed on the word lines and the erase gate to produce problems of short circuit and current leakage in a later chemical mechanical polishing process.

PRIORITY CLAIM AND CROSS-REFERENCE

The present application is a divisional of U.S. application Ser. No. 14/229,191, filed on Mar. 28, 2014, the disclosure of which are hereby incorporated by reference herein in its entirely.

BACKGROUND

The functionality and performance of an advanced logic circuit for mobile applications can be further enhanced by embedding nonvolatile memory with the advanced logic circuit. However, some problems still need to be solved to integrate a process of a nonvolatile memory with an advanced logic circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A-1H are cross-sectional diagrams showing a process of embedded nonvolatile memory according to some embodiments of this disclosure.

FIGS. 2A-2D are cross-sectional diagrams showing a process of embedded nonvolatile memory according to some other embodiments of this disclosure.

FIGS. 3A-3D are cross-sectional diagrams showing a process of embedded nonvolatile memory according to some other embodiments of this disclosure.

The drawings, schematics and diagrams are illustrative and not intended to be limiting, but are examples of embodiments of the disclosure, are simplified for explanatory purposes, and are not drawn to scale.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

One problem of integrating nonvolatile memory process with an advanced logic process for mobile applications is caused by forming metal silicide on the top of the word lines and erase gates in the nonvolatile memory area when metal silicide is formed on the source/drain regions of the logic area. After chemical mechanical polishing process, the metal silicide on the top of the word lines will be spread over the entire wafer and problems of short circuit and current leakage are thus produced. Therefore, this disclosure provides a novel process of manufacturing nonvolatile memory that can be integrated with an advanced logic process for mobile applications to solve the problem above. According various embodiments of this disclosure, the nonvolatile memory may be a stacked gate memory.

FIGS. 1A-1H are cross-sectional diagrams showing a process of embedded nonvolatile memory according to some embodiments of this disclosure. In FIGS. 1A-1H, the word lines 122 a and the erase gates 122 b of the nonvolatile memory formed in FIG. 1E will be etched back in FIG. 1F. Thus, the top surfaces of the word lines 122 a and the erase gates 122 b is not higher than the top surfaces of the control gates 108 to avoid forming metal silicide on the top surfaces of the word lines 122 a and the erase gates 122 b.

In FIG. 1A, a tunneling oxide layer 102 and a first polysilicon layer are sequentially formed on a substrate 100, which has isolation structures (not shown in FIG. 1A) formed therein. The first polysilicon layer is then patterned to form plural polysilicon stripes 104 paralleling to the surface of the paper. The isolation structures above may be shallow trench isolations (STIs), for example. The tunneling oxide layer 102 may be formed by a thermal oxidation process. The first polysilicon layer may be formed by chemical vapor deposition and then be patterned by photolithography and followed by dry etching, for example, to form the plural polysilicon stripes 104. The first polysilicon may have a thickness of 150-300 Å, such as 200 Å.

Next, a middle dielectric layer 106, a second polysilicon layer, and a first dielectric layer are sequentially formed above the substrate 100 to cover the plural polysilicon stripes 104 and the tunneling oxide layer 102. The middle dielectric layer 106 may include a bottom silicon oxide layer, a middle silicon nitride layer, and a top silicon oxide layer, for example. The bottom and top silicon oxide layers may be formed by thermal oxidation at a temperature of 800-1200° C. followed by annealing at 1000° C., and may have a thickness of 40 Å, for example. The middle silicon nitride layer may be formed by low pressure chemical vapor deposition (LPCVD), and may have a thickness of 80 Å, for example. The second polysilicon layer may be formed by chemical vapor deposition and have a thickness of 300-600 Å, such as 250 Å. The first dielectric layer may be made of silicon nitride deposited by LPCVD and have a thickness of 1000-1500 Å, such as 1300 Å.

Then, the first dielectric layer and the second polysilicon layer are patterned to form mask layers 110 and control gates 108, respectively. The patterning method may be performed by photolithography and followed by dry etching. During the etching of the second polysilicon layer, the mask layers 110 are used as an etching mask.

In FIG. 1B, a second dielectric layer is formed above the substrate 100 to conformally cover the mask layers 110, the control gates 108 and the middle dielectric layer 106. Next, the second dielectric layer is anisotropically etched to form first spacers 112 on the sidewalls of the mask layers 110 and the control gates 108. Subsequently, the exposed middle dielectric layer 106, the plural polysilicon stripes 104 and the tunneling oxide layer 102 thereunder are etched to form the middle dielectric layer 106 a, the floating gate 104 a, and the tunneling oxide layer 102 a to form gate stacks 114 on the substrate 100. The second dielectric layer may include a bottom silicon oxide layer, a middle silicon nitride layer, and a top silicon oxide layer.

In FIG. 1C, a first buffer layer 116 a and a third dielectric layer are sequentially formed above the substrate 100 to conformally cover the exposed surfaces of the gate stacks 114 and the exposed substrate 100. The first buffer layer 116 a may be a silicon oxide layer formed by chemical vapor deposition, for example. The third dielectric layer may be a silicon nitride layer formed by LPCVD, for example. The third dielectric layer is then anisotropically etched to form second spacers 118 on sidewalls of the gate stacks 114. The anisotropic etch may be performed by dry etching.

The first buffer layer 116 a above is usually used to release the strains caused by lattice mismatch between the third dielectric layer and the exposed silicon layers when the lattice mismatch above is obvious. For example, the exposed silicon layers include floating gates 104 a and the substrate 100 in FIG. 1C. However, if the lattice mismatch between the third dielectric layer and the exposed silicon layers is not so obvious to generate obvious strains, the first buffer layer 116 a may be omitted.

Next, a patterned photoresist layer 121 is formed by a combination of spin coating, exposing and developing processes to expose the common source area of the substrate 100. Ions are then implanted into the exposed substrate 100 to form the common source 120. Subsequently, the second spacers 118 exposed by the patterned photoresist layer 121 is removed, and the removal method may be performed by dry etching or wet etching, for example. During the removal of the exposed second spacers 118, the first buffer layer 116 a may be consumed finally to expose the common source 120.

In FIG. 1D, the patterned photoresist layer 121 is removed, and the removal method may be performed by solvent stripping or plasma ashing, for example. Then, a gate oxide layer 116 b is formed to cover the exposed substrate 100, i.e. the common source 120. The gate oxide layer 116 b may be formed by thermal oxidation.

Next, a third polysilicon layer 122 and a fourth dielectric layer 124 are sequentially formed above the substrate 100. The thickness of the third polysilicon layer 122 is smaller than a total thickness of the tunneling oxide layer 102 a, the floating gate 104 a, the middle dielectric layer 106 a, and the control gates 108, such as in a range from about 400 Å to about 600 Å. The thickness of the fourth dielectric layer 124 is in a range from about 200 Å to about 400 Å. The forth dielectric layer 124 may be made of silicon oxide formed by LPCVD, for example.

In FIG. 1E, the fourth dielectric layer 124 is anisotropically etched to form first side cap layers 124 a on the word lines 122 a and first middle cap layers 124 b on the erase gates 122 b, and the etching is stopped on the third polysilicon layer 122. Subsequently, the exposed third polysilicon layer 122 is anisotropically etched to form word lines 122 a next to the second spacers 118 and erase gate 122 b above the common source 120, and the etching is stopped on the first buffer layer 116 a. The anisotropic etching above may be performed by dry etching.

Pleased noted that since the first buffer layer 116 a is quite thin, and thus the exposed portions of the first buffer layer 116 a may be easily etched away to expose the substrate 100 thereunder during the etching of the third polysilicon layer 122. Therefore, an organic material is spin coated above the substrate 100 to form an organic layer 126 covering the exposed top surface of the substrate 100 to protect the exposed substrate 100. Simultaneously, since the exposed top surfaces of the word lines 122 a and the erase gate 122 b are etched to have a concave top surface, the organic material also can be spin coated on the top surfaces of the word lines 122 a and the erase gate 122 b. In addition, the thickness of the organic layer 126 covering the substrate 100 is more than the thickness of the organic layer 126 covering the word lines 122 a and the erase gate 122 b to provide a better protection to the substrate 100. The organic material above may be photoresist or other organic polymers that can be spin-coated to protect the exposed substrate 100 during the subsequent etching of the word lines 122 a and the erase gate 122 b.

In FIG. 1F, the organic layer 126 is etched to expose the top surfaces of the word lines 122 a and the erase gate 122 b, but the top surface of the substrate 100 is still covered by the organic layer 126. Next, the exposed word lines 122 a and the exposed erase gate 122 b are selectively etched back by isotropic dry etching to avoid damaging the second spacers 118, the first side cap layers 124 a and the first middle cap layers 124 b made of silicon nitride. Therefore, the level of the top surfaces of the word lines 122 a and the erase gate 122 b are lowered. Then, the remained organic layer 126 on the substrate 100 is removed by solvent stripping, for example.

According to some embodiments, the isotropic dry etching above may be performed by an inductively-coupled plasma (ICP) poly etcher. The source of the etching plasma may include a mixture of 5-50 sccm of SF₆ and 100-600 sccm of a carrier gas, and the carrier gas may be Ar or He. The pressure in the reactive chamber may be increased to 3-50 mTorr, and the ICP power may be increased to 200-600 W. In addition, the bias voltage may be decreased to 0-100 V. Since SF₆ is used as the source of the etching plasma, the dry etching can be isotropic.

According to some other embodiments, the dry etching above may be performed by a chemical dry etcher. The chemical dry etcher equipped with a remote plasma source to decrease the kinetic energy of the generated plasma to almost zero. Therefore, an isotropic etching can be performed to decrease the damage caused by high kinetic energy plasma. In the chemical dry etching (CDE) process, the source of the plasma may include a mixture of C_(x)H_(y)F_(z) and oxygen. The total flow rate of the mixture gas may be 300-800 sccm, and the flow rate ratio of the C_(x)H_(y)F_(z) to oxygen may be 0.5-1.5. The C_(x)H_(y)F_(z) may be CH₂F₂, CHF₃, CF₄, C₂F₆, C₃F₈, C₄F₆, or C₅F₈. The pressure of the reactive chamber may be 200-500 mTorr. The etching selectivity of silicon over silicon nitride is about 3-10, and thus the damage of the second spacers 118, the first side cap layers 124 a and the first middle cap layers 124 b may be effectively decreased.

In FIG. 1G, a second buffer layer 128 and a fifth dielectric layer are sequentially formed above the substrate 100 to cover the structures on the substrate 100. The fifth dielectric layer is then anisotropically etched to form third spacers 130 a on sidewalls of the word lines 122 a, second side cap layers 130 b on the top surface of the word lines 122 a and second middle cap layers 130 c on the erase gate 122 b. At the same time, the exposed second buffer layer 128 is also etched away during the etching of the fifth dielectric layer, since the second buffer layer 128 is kind of thin. The second buffer layer 128 may be a silicon oxide layer formed by CVD. The fifth dielectric layer may be a silicon nitride layer formed by LPCVD. The anisotropic etching may be performed by dry etching, for example. Similarly, the second buffer layer 128 may be omitted when the lattice mismatch between the fifth dielectric layer and the exposed silicon layers is not obvious to create obvious strain.

Subsequently, a self-aligned silicidation (salicide) process is performed to form metal silicide on exposed surfaces of silicon material on both the nonvolatile memory area and the logic area. Therefore, metal silicide will be formed on the exposed surfaces of the substrate 100 and other polysilicon layers. Please note that since the exposed surfaces of the word lines 122 a and the erase gate 122 b have been covered by the second buffer layer 128, first side cap layers 124 a, first middle cap layers 124 b, third spacers 130 a, second side cap layers 130 b, and second middle cap layer 130 c, no metal silicide can be formed on the top surface of the word lines 122 a and the erase gate 122 b. In the nonvolatile memory area, metal silicide layers 132 can be formed only on the exposed surfaces of the substrate 100 to be used as drains.

In FIG. 1H, an etching stop layer 134 is formed above the substrate 100 to conformally covered the structures on the substrate 100. The material of the etching stop layer 134 may be silicon nitride formed by LPCVD, for example. Then, a low-k dielectric layer 136 is formed above the substrate 100 to cover the structures formed on the substrate 100. A process of chemical mechanical polishing (CMP) is subsequently performed to polish the whole wafer to remove an upper portion of the low-k dielectric layer 136, and the CMP is stopped on the mask layer 110. Hence, the thickness of the mask layer 110 is decreased further.

The material of the low-k dielectric layer 136 may be made from a dielectric material having a dielectric constant smaller than the dielectric constant of silicon dioxide (i.e. a low-k dielectric material). Common low-k dielectric material includes fluorine-doped silicon dioxide, carbon-doped silicon dioxide, porous silicon dioxide, porous carbon-doped silicon dioxide, a spin-on organic polymeric dielectric (such as polyimide, polynorbornenes, benzocyclobutene, or polytetrafluoroethylene), a spin-on silicone based polymeric dielectric (such as hydrogen silsesquioxane (HSQ) and methylsilsesquioxane (MSQ)).

FIGS. 2A-2D are cross-sectional diagrams showing a process of embedded nonvolatile memory according to some other embodiments of this disclosure. Since the processes before FIG. 2A are similar to FIGS. 1A-1C, the figures and the detailed descriptions are omitted here. In addition, the reference numbers in FIG. 2A representing the same or similar components are obtained by adding 100 to the reference numbers in FIG. 1C, and the meanings of the reference numbers in FIG. 2A representing the same or similar components are thus not described repeatedly. In FIGS. 2A-2D, the third polysilicon layer 122 and the fourth dielectric layer 124 in FIG. 1D is replaced by only a third polysilicon layer 222 in FIG. 2A, and the third polysilicon layer 222 is etched to form the word lines 222 a and the erase gate 222 b having top surfaces not higher than the top surfaces of the control gate 208 in FIG. 2B to avoid forming metal silicide on the top surfaces of the word lines 222 a and the erase gates 222 b. The detailed descriptions of FIGS. 2A-2D are described below.

After the removal of the photoresist layer 121 in FIG. 1C, the exposed first buffer layer 216 a is then removed in FIG. 2A. The removal method of the exposed first buffer layer 216 a may be wet etching, for example. A gate oxide layer 216 b is grown to cover the exposed surfaces of the substrate 200, the floating gate 204 a and the common source 220. The formation method of the gate oxide layer 216 b may be performed by thermal oxidation. Next, a third polysilicon layer 222 is formed to cover the substrate 200, and the thickness of the third polysilicon layer 222 is greater than the total thickness of the gate stacks 214. According to some embodiments, the thickness of the third polysilicon layer 222 is in a range from about 1800 Å to about 2200 Å.

In FIG. 2B, the third polysilicon layer 222 is anisotropically etched until the substrate 200 is exposed to form word lines 222 a and erase gate 222 b. Then, a second buffer layer 224 and a fourth dielectric layer are sequentially formed above the substrate 200. The fourth dielectric layer is anisotropically etched to form side cap layers 226 a on the word lines 222 a and first middle cap layers 226 b on the erase gates 222 b, and the exposed second buffer layer 224 is consumed during the etching of the fourth dielectric layer. Next, the exposed word lines 222 a are further etched by using the side cap layers 226 a as an etching mask to modify the profile of the word lines 222 a. As for the erase gate 222 b, since the fourth dielectric layer is thicker over the erase gate 222 b, the erase gate 222 b is finally not etched during the etching back of the word lines 222 a.

In FIG. 2C, a third buffer layer 228 and a fifth dielectric layer are sequentially formed above the substrate 200. The fifth dielectric layer is anisotropically etched to form third spacers 230 a on sidewalls of the word lines 222 a and a second middle cap layer 230 b on the erase gates 222 b. The exposed third buffer layer 228 is consumed during the etching of the fifth dielectric layer. The third buffer layer 228 may be a silicon oxide layer formed by CVD. The fifth dielectric layer may be a silicon nitride layer formed by LPCVD. Similarly, the third buffer layer 228 may be omitted if the stress between the fifth dielectric layer and the exposed silicon layers is not too much.

Then, a self-aligned silicidation (salicide) process is performed to form metal silicide on exposed surfaces of silicon material on both the nonvolatile memory area and the logic area. Therefore, the exposed surface of the substrate 200 and other polysilicon layers will have metal silicide 232 formed thereon. Please note that since the top surfaces of the word lines 222 a and the erase gate 222 b are not exposed, no metal silicide can be formed on the top of the word lines 222 a and the erase gate 222 b.

In FIG. 2D, an etching stop layer 234 is formed above the substrate 200 to conformally covered the structure on the substrate 200. The material of the etching stop layer 234 may be silicon nitride, for example. Then, a low-k dielectric layer 236 is formed above the substrate 200 to cover the structures formed on the substrate 200. A process of chemical mechanical polishing (CMP) is subsequently performed to polish the whole wafer to remove an upper portion of the low-k dielectric layer 236, and the CMP is stopped on the mask layer 210. Hence, the thickness of the mask layer 210 is decreased further. The material of the low-k dielectric layer 236 is similar to the material of the low-k dielectric layer 136, and hence omitted here.

FIGS. 3A-3D are cross-sectional diagrams showing a process of embedded nonvolatile memory according to some other embodiments of this disclosure. Since the processes before FIG. 3A are similar to FIGS. 1A-1C, the figures and the detailed descriptions are omitted here. In addition, the reference numbers in FIG. 3A representing the same or similar components are obtained by adding 200 to the reference numbers in FIG. 1C, and the meanings of the reference numbers in FIG. 3A representing the same or similar components are not described repeatedly. In FIGS. 3A-3D, the third polysilicon layer 222 in FIG. 2A is replaced by a third polysilicon layer 322 and an organic layer 324 in FIG. 3A. Therefore, the organic layer 324 and the third polysilicon layer 322 are non-selectively etched back to leave the third polysilicon layer 322 having a top surface not higher than the top surface of the control gate 308 in FIG. 3B. Thus, metal silicide can be avoided to be formed on the top surfaces of the word lines 322 a and the erase gate 322 b. The detailed descriptions of FIGS. 3A-3D are described below.

After the removal of the photoresist layer 121 in FIG. 1C, the exposed first buffer layer 316 a is then removed in FIG. 3A. The removal method of the exposed first buffer layer 316 a may be wet etching, for example. A gate oxide layer 316 b is grown to cover the exposed surfaces of the substrate 300, the floating gate 304 a and the common source 320. The formation method of the gate oxide layer 316 b may be performed by thermal oxidation. Next, a third polysilicon layer 322 and an organic layer 324 is formed to cover the substrate 300. The thickness of the third polysilicon layer 322 is smaller than a total thickness of the tunneling oxide layer 302 a, the floating gate 304 a, the middle dielectric layer 306 a, and the control gates 308, such as in a range from about 400 Å to about 600 Å. The top surface of the organic layer 324 is higher than the top surfaces of the gate stacks 314. Therefore, the thickness of the organic layer 324 may be in a range from about 1000 Å to about 1500 Å according to some embodiments.

In FIG. 3B, the third polysilicon layer 322 and the organic layer 324 are non-selectively etched until the top surfaces of the third polysilicon layer 322 is lower than the top surfaces of the control gates 308. According to some embodiments, the thickness of the remained third polysilicon layer 322 is in a range from about 600 Å to about 800 Å. In this step, an erase gate 322 b is formed. Then, the residue of the organic layer 324 is removed, and the removal may be performed by plasma ashing.

Next, a second buffer layer 326 and a fourth dielectric layer are sequentially formed above the substrate 300 to cover the structures on the substrate 300. The fourth dielectric layer is anisotropically etched to form side cap layers 328 a on the word lines 322 a and a first middle cap layer 328 b on the erase gate 322 b, and some of the exposed second buffer layer 326 is consumed during the etching of the fourth dielectric layer. Next, the exposed third polysilicon layer 322 is further etched by using the side cap layers 328 a as an etching mask to form word lines 322 a. The second buffer layer 326 may be a silicon oxide layer formed by CVD. The fourth dielectric layer may be a silicon nitride layer formed by LPCVD. Similarly, the second buffer layer 326 may be omitted when the strains between the fourth dielectric layer and the exposed silicon layer is not too much.

In FIG. 3C, a third buffer layer 330 and a fifth dielectric layer are sequentially formed above the substrate 300. The fifth dielectric layer is anisotropically etched to form third spacers 332 a on sidewalls of the word lines 322 a and a second middle cap layer 332 b on the erase gate 322 b. The exposed third buffer layer 330 is consumed during the etching of the fifth dielectric layer. The third buffer layer 330 may be a silicon oxide layer formed by CVD. The fifth dielectric layer may be a silicon nitride layer formed by LPCVD. Similarly, the third buffer layer 330 may be omitted when the strains between the fifth dielectric layer and the exposed silicon layer is not too much.

Then, a self-aligned silicidation (salicide) process is performed to form metal silicide 334 on exposed surfaces of silicon material on both the nonvolatile memory area and the logic area. Therefore, the exposed surface of the substrate 300 and other polysilicon layers will have metal silicide 334 formed thereon. Please note that since the top surfaces of the word lines 322 a and the erase gate 322 b are not exposed, no metal silicide can be formed on the top of the word lines 322 a and the erase gate 322 b.

In FIG. 3D, an etching stop layer 336 is formed above the substrate 300 to conformally covered the structure on the substrate 300. The material of the etching stop layer 336 may be silicon nitride, for example. Then, a low-k dielectric layer 338 is formed above the substrate 300 to cover the structure formed on the substrate 300. A process of chemical mechanical polishing (CMP) is subsequently performed to polish the whole wafer to remove an upper portion of the low-k dielectric layer 338, and the CMP is stopped on the mask layer 310. Hence, the thickness of the mask layer 310 is decreased further. The material of the low-k dielectric layer 338 is similar to the material of the dielectric layer 136, and hence omitted here.

Accordingly, this disclosure provides three different method to lower the top surfaces of the word lines and erase gates, hence the word lines and erase gates can have top surfaces lower than the top surfaces of the control gates. Furthermore, dielectric cap layers are formed on top surfaces of the word lines and the erase gates, and dielectric spacers are formed on sidewalls of the word lines. Therefore, no surfaces of the word lines and erase gates are exposed when self-aligned silicidation process is performed on both the nonvolatile memory area and the 28 HPM logic area, and no metal silicide can be formed on the word lines and erase gate. Consequently, during the CMP process, no metal silicide can be spread out to produce problems of current leakage and short circuits.

According to some embodiments of this disclosure, a nonvolatile memory is provided, and the nonvolatile memory comprises the following components. At least two gate stacks are located on a substrate, wherein the gate stacks each from bottom to top sequentially comprises a tunneling oxide layer, a floating gate, a middle dielectric layer, a control gate, and a mask layer. First spacers are located on sidewalls of the two gate stacks. A gate dielectric layer located on the exposed substrate. An erase gate is located between the two gate stacks and has a nonplanar top surface not higher than top surfaces of the control gates. Two word lines are located on outer sides of the two gate stacks and have nonplanar top surfaces not higher than the top surfaces of the control gates. Cap layers are located respectively on the erase gate and the word lines.

According to some other embodiments of this disclosure, a nonvolatile memory is provided, and nonvolatile memory comprises the following components. At least two gate stacks are located on a substrate, wherein the gate stacks each from bottom to top sequentially comprises a tunneling oxide layer, a floating gate, a middle dielectric layer, a control gate, and a mask layer. First spacers are located on sidewalls of the two gate stacks. A gate dielectric layer located on the exposed substrate. An erase gate is located between the two gate stacks and has a curved top surface lower than top surfaces of the control gates. Two word lines have curved top surfaces lower than the top surfaces of the control gates. One of the gate stacks is located between one of the word lines and the erase gate. Cap layers are located respectively on the erase gate and the word lines.

According to some other embodiments of this disclosure, a method of forming a nonvolatile memory is provided. Two gate stacks are formed on a substrate, wherein the gate stacks each from bottom to top sequentially comprises a tunneling oxide layer, a floating gate, a middle dielectric layer, a control gate, and a mask layer. First spacers are formed on sidewalls of the two gate stacks. A gate dielectric layer is formed on the exposed substrate. An erase gate between the two gate stacks and two word lines located on outer sides of the two gate stacks are simultaneously formed, wherein the erase gate and the two word lines have top surfaces not higher than top surfaces of the control gates. Composite cap layers are formed respectively on the top surfaces of the erase gate and the word lines.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A nonvolatile memory, comprising: at least two gate stacks located on a substrate, wherein the two gate stacks each from bottom to top sequentially comprise: a tunneling oxide layer, a floating gate, a middle dielectric layer, a control gate, and a mask layer; first spacers located on sidewalls of the two gate stacks; a gate dielectric layer located on an exposed surface of the substrate; an erase gate located between the two gate stacks and having a nonplanar top surface not higher than top surfaces of the control gates; two word lines located on outer sides of the two gate stacks and having nonplanar top surfaces not higher than the top surfaces of the control gates; cap layers located respectively on the erase gate and the word lines; and a first buffer layer disposed between the erase gate and at least one of the cap layers.
 2. The nonvolatile memory of claim 1, wherein the top surfaces of the word lines are U shape.
 3. The nonvolatile memory of claim 1, wherein the erase gate and the word lines are made from polysilicon.
 4. The nonvolatile memory of claim 1, wherein the first spacers and the cap layers each comprise an inner silicon oxide layer and an outer silicon nitride layer.
 5. The nonvolatile memory of claim 1, further comprising second spacers located on outer sidewalls of the word lines.
 6. The nonvolatile memory of claim 5, wherein the second spacers each comprise an inner silicon oxide layer and an outer silicon nitride layer.
 7. The nonvolatile memory of claim 5, further comprising a second buffer layer disposed between one of the second spacers and one of the word lines.
 8. The nonvolatile memory of claim 5, further comprising an etching stop layer located on outer sidewalls of the second spacers.
 9. The nonvolatile memory of claim 8, wherein the etching stop layer is made from silicon nitride.
 10. The nonvolatile memory of claim 8, further comprising a low-k dielectric layer covering the etching stop layer.
 11. The nonvolatile memory of claim 10, wherein the low-k dielectric layer is made from one or more of the following: fluorine-doped silicon dioxide, carbon-doped silicon dioxide, porous silicon dioxide, porous carbon-doped silicon dioxide, a spin-on organic polymeric dielectric, and a spin-on silicone based polymeric dielectric.
 12. The nonvolatile memory of claim 1, further comprising: a source located in the substrate and between the two gate stacks.
 13. The nonvolatile memory of claim 12, further comprising: a drain located in the substrate, wherein one of the gate stacks is located between the drain and the source.
 14. The nonvolatile memory of claim 1, wherein a top surface of at least one of the cap layers and a top surface of one of the gate stacks are substantially coplanar.
 15. The nonvolatile memory of claim 1, wherein the erase gate and the word lines are made from substantially the same material.
 16. The nonvolatile memory of claim 1, wherein one of the cap layers abuts the erase gate.
 17. The nonvolatile memory of claim 1, wherein the nonplanar top surfaces of the word lines are concave surfaces.
 18. The nonvolatile memory of claim 1, wherein the nonplanar top surfaces of the word lines are convex surfaces.
 19. The nonvolatile memory of claim 1, further comprising a second buffer layer disposed between one of the word lines and at least one of the cap layers.
 20. The nonvolatile memory of claim 1, wherein the first buffer layer is made from silicon oxide. 